With the use of thermoplastic materials for sealing corrosive environments considerable difficulties have been encountered in maintaining the seal when it is exposed to substantial variations in temperature. For example, a polytetrafluoroethylene (PTFE) seal which is exposed to temperature which varies from +300.degree. F. down to -80.degree. F., has a substantially larger coefficient of internal expansion than the metal bodies against which they seal so that prior seals have been unsatisfactory in such service.
The following U.S. patents disclose seals which may be pertinent with respect to the present invention:
U.S. Pat. No. 3,269,736 discloses a seal for cryogenic use having a first PTFE ring seal and a second ring spring of "Mylar" which like PTFE remains flexible at extremely low temperatures but also is more resilient at low temperatures to provide a spring action. U.S. Pat. No. 4,199,158 discloses a tunnel seal for use with concrete blocks. U.S. Pat. No. 4,201,310 discloses a bidirectional seal for the cover of a petrol drum. U.S. Pat. Nos. 4,371,177, 4,365,818, 4,034,959, 3,970,321, 3,829,106, 3,827,671 and 3,356,333 disclose a wide variety of shapes of seals suggested by the prior art.